This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In biological membranes, when lo + ld phases coexist ("rafts"), the size of the domains seems to be much smaller than the wavelength of light, i.e. at least one dimension of size <300 nm. In chemically-defined mixtures of SM/POPC/chol, or DSPC/POPC/chol, or DSPC/SOPC/chol, giant unilamellar vesicles (GUVs) appear uniform in fluorescence microscopy over the entire composition region where lipid rafts could occur. However, FRET and single dye fluorescence studies indicate phase separation at well-defined boundaries. By use of a spin-labeled phospholipid, "16-PC", we examine the hyperfine splitting of samples across the putative phase boundaries in DSPC/POPC/chol, and compare with the hyperfine splitting across phase boundaries in DSPC/DOPC/chol, where large phase domains are observed in GUVs.